Big Ben is probably the most famous landmark in London, but did you know that you can actually get permission to climb the clock tower? A tour will take you up all 334 steps to the belfry, where you'll stand ten feet away from the Big Ben bell as it strikes the hour (don't worry, earplugs are provided!).
You will also be able to enjoy spectacular views of Westminster and the river from 62 metres up, and have an opportunity to stand behind all four clock faces and visit the pendulum and mechanism, to learn about how it all works.
Unfortunately, it's a lot harder to get into Big Ben than it is to get into Parliament itself. You can't just turn up on the day and ask for a tour -- you have to jump through a few hoops first.
First of all, you have to be a UK resident, and you must obtain a ticket by writing to your local MP. (Work out who your MP is by looking here.) They will undertake a security check on your personal details before giving you a slot. You should be prepared for a long wait -- tours are often booked up 3 or 4 months in advance. You should also expect tight security on the day -- you will be expected to bring two forms of ID and have your photo taken. Photography will not be allowed inside the tower.